Section 3: Polygenic and Multiple Alleles

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Unit 5: Macrogenetics

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Chapter 12.3

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Objectives

• Describe and predict multiple alleles and polygenic patterns of inheritance.

Warm-up:

In humans, there are many traits that don’t seem to follow simple dominant/recessive, incomplete dominance, or co-dominance patterns of inheritance. What are some of these traits?

Multiple Alleles

• The traits we have been looking at so far are all controlled by one gene that has two different alleles.

Multiple Alleles

• Some traits are passed on with just one gene, but have more than two alleles.
• Multiple Alleles: more than two forms of a gene control the expression of a trait.
• Ex: human blood types

Multiple Alleles

• Humans can have four possible blood types: A, B, AB, or O

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Type A

Has A antigens

Type B

Has B antigens

Type AB

Has A and B antigens

Type O

Has no antigens

Multiple Alleles

• Blood types are controlled by 3 possible alleles.

Multiple Alleles

• The A and B alleles are co-dominant to each other, and the O allele is recessive to both A and B.

Multiple Alleles

• Because multiple alleles control blood type, we need a different way to write the genotypes.
• I^A - produces the A antigen
• I^B - produces the B antigen
• i - does not produce an antigen

3 Alleles

Capital I for the A antigen and B antigen because they are both dominant

Lowercase i for no antigens because it is recessive

Multiple Alleles

• Because of the possible combinations of alleles, there are four different blood types:

Type A

I^AI^A

I^Ai

Type B

I^BI^B

I^Bi

Type AB

I^AI^B

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Type O

ii

I^B

i

i

I^Bi

I^Ai

ii

I^AI^B

I^A

1/4 = I^AI^B

1/4 = I^Bi

1/4 = I^Ai

1/4 = ii

1/4 = AB

1/4 = A

1/4 = B

1/4 = O

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C = agouti

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c^ch = chinchilla (silver)

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c^h = Himalayan

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c^a = albino

Polygenic Inheritance

• It is also possible to have a trait that is controlled by more than one gene.
• Polygenic Trait: a trait in which the phenotype is controlled by two or more genes at different locations (loci) on different chromosomes.

Polygenic Inheritance

• Polygenic traits usually show a spectrum of different phenotypes.
• Examples: skin color, eye color, height

Polygenic Inheritance

• With polygenic traits, each gene may have different alleles.
• When specific alleles from different genes are combined, they may affect the phenotype in many different ways.

The “B” gene determines the amount of pigment.

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The “E” gene determines whether or not there will be eumelanin (dark pigment).

These are two sets of twins. How is this possible?

Phenotypes

Number of dark skin alleles

Fingerprints

Amoeba Sisters ABO Blood Types